Superalloys are traditionally subdivided according to whether strength is obtained from solution hardening or the precipitation of secondary phases. The present invention is directed to Ni or Fe-based austenitic (FCC) precipitation hardened alloys, specifically, alloys in which precipitation hardening is derived from (1) the presence of carbide forming agents such as: Nb, Cr, Co, Mo, W, Ta, and V, as well as (2) intermetallic compounds formed by Al and Ti at concentrations typically ranging between 1% and 5%. With the exception of Cr, carbide formers usually exist in concentrations of less than 5%.
Examples of the nominal compositions of selected commercially significant Ni- and Fe-based, precipitation hardened, superalloys are provided in Table 1. (It should be noted that the scope of alloy compositions to which the processes described herein applies includes, but is not necessarily limited to those listed in Table 1). All footnoted references herein are to be taken as incorporated by reference in the specification for their respective disclosures and teachings concerning superalloys and background metallurgical science.
TABLE 1 ______________________________________ Alloy Composition in wt % Designation Ni Fe Cr Co Al Ti Mo Other ______________________________________ Alloy V-57 26 bal 15 -- 0.25 3 1.25 0.3 V Alloy 738 bal -- 16 -- 3.5 3.5 1.8 2.6 W, 0.9 Nb Alloy 100 bal -- 10 15 5.5 4.7 3 0.95 V Alloy 939 bal -- 23 19 1.9 3.7 -- 2 W, 1 Nb, 1.4 Ta ______________________________________
The alloying additions to the Ni and Fe-based superalloys of Table 1, whether in solid solution or precipitate form, allow the tensile strength of these materials to be maintained at temperatures in excess of 80% of the melting point.sup.i. As a result, these materials have become widely used in high temperature applications such as: nuclear reactors, petrochemical equipment, submarines and rocket/jet and gas turbine engines.sup.1-4.
In many of the industrial applications cited above, these materials are required to reliably sustain temperatures and stresses in excess of 1000.degree. C. and 400 MPa, respectively for periods of up to 10,000 hours.sup.2. Further, stress and temperature extremes are often accompanied by exposure to sulphate and other corrosive media. Under these conditions, reliability, and service life of superalloy components is contingent upon resistance to creep, intergranular corrosion, and fatigue.sup.1-3. Sustained temperatures of between 800.degree. C. and 1000.degree. C. (in the presence of sulfur, which diffuses along grain boundaries forming Ni.sub.3 S.sub.2, CrS or Cr.sub.2 S.sub.3, commonly referred to as "spiking"), render these alloys susceptible to intergranular degradation by "hot" corrosion, fatigue, and creep. "Hot corrosion" and sulfide "spiking" at intergranular cites ultimately results in a loss of tensile, fatigue, and impact strength.sup.1-4.
Moreover, Ni-and Fe-based precipitation hardened superalloys such as: Alloy V-57, Alloy 738, and Alloy 100 generally exhibit poor weldability, limiting their use in applications where complex geometries are constructed by joining of individual components. For example, this has been the main limitation for using higher temperature precipitation-strengthened alloy formulations for combustor-can components.sup.2. Weldability correlates directly with the Al and Ti content in the alloy, as illustrated in FIG. 1.sup.5. Gamma prime (.gamma.') phases formed by these constituents (i.e. Ni.sub.3 (Al,Ti)) which are responsible for high temperature strength, precipitate along grain boundaries in the weld heat-affected-zones resulting in hot cracking (during welding) and Post-Weld Heat Treatment (PWHT) cracking.
Although significant improvements have been made in minimizing these intergranular effects by alloying additions to control the content, distribution, and growth (Oswald ripening) of intermetallic .gamma.' (NiAl.sub.3) and carbide (MC, M.sub.23 C.sub.6 MKC) phases.sup.6,7, thermal conductivity and phase stability place practical limits on alloying as a means of further improving corrosion, creep, fatigue, and strength performance. Single crystal, directionally solidified, ceramic, and diffusion barrier overlay components such as NiAl.sub.3 or MCrAlY offer superior fatigue, corrosion, and creep resistance than conventional superalloys, largely at the expense of cost, manufacturing throughput, and often reliability.sup.2,4,7. Fracture toughness and critical defect sizes in competing materials such as ceramics (eg. silicon nitride) are approximately two orders of magnitude smaller than for nickel-based superalloys at typical operating stresses.sup.2, significantly limiting reliability of these high temperature materials.sup.2.
It has been shown that grain boundaries having misorientations described on the basis of the Coincident Site Lattice Model (CSL).sup.8 of interface structure as lying within .DELTA..theta. of .SIGMA. where .SIGMA..ltoreq.29 and .DELTA..theta..ltoreq.15.SIGMA..sup.1/2 9 are highly resistant to intergranular degradation processes such as: corrosion.sup.10, cracking.sup.11, and grain boundary sliding/cavitation.sup.12-14. This arises from the reduced free volume and superior fit between the abutting lattices that form boundaries between adjacent grains in the microstructure. The present applicants have previously disclosed that the frequency of these degradation-resistance grain boundaries can be enhanced in the microstructure of various FCC materials including lead.sup.15,16 and austenitic stainless alloys.sup.17 from 10%-20% to levels in excess of 50% to 60% resulting in significant improvements in creep, intergranular corrosion, and cracking resistance.
Evidence exists to suggest that high fractions of "special" grain boundaries can stabilize passive oxide layers, while significantly reducing localized grain boundary attack.sup.18. Solution hardened Alloys 600 and 800 processed such that 80% of the grain boundaries in the microstructure are "special" have been previously demonstrated by the present applicants to be virtually immune to intergranular corrosion.sup.10. In addition, we have recently demonstrated that microstructures of pure nickel having "special" grain boundary fractions in excess of 50% exhibit improvements of 15 fold and 5 fold in steady-state creep rate and primary creep strain, respectively.sup.19. Furthermore, the reduced propensity for solute segregation, cracking, and cavitation, offers the potential for minimizing alloy susceptibility to crack nucleation and propagation originating from low-cycle fatigue and Post Weld Heat Treatment (PWHT) cracking.sup.2,3. In contrast to traditional alloy development approaches wherein treatments applied to benefit one characteristic often degrade other performance aspects, optimizing grain boundary structure in these superalloys provides for simultaneously improving creep, corrosion, fatigue, and weldability performance. Furthermore, since altering grain boundary structure does not necessarily involve variations in alloy chemistry, improvements in performance cannot detrimentally affect thermal conductivity and phase stability.